Scanning electron beam computed tomography ("CT") systems are described generally in U.S. Pat. Nos. 4,352,021 to Boyd, et al. (Sep. 28, 1982), and 4,521,900 (Jun. 4, 1985), 4,521,901 (Jun. 4, 1985), 4,625,150 (Nov. 25, 1986), 4,644,168 (Feb. 17, 1987), and 5,193,105 (Mar. 9, 1993), all to Rand, et. al. Applicant refers to and incorporates herein by reference each above listed patent to Rand, et al.
As described in the above-referenced patents, an electron beam is produced by an electron gun at the upstream end of an evacuated generally conical shaped housing chamber. A large electron gun potential (e.g., 130 kV) accelerates the electron beam downstream along the chamber axis, and further downstream, a beam optical system focuses and deflects the beam to scan along an X-ray producing target. It is understood that the final beam spot on the target is much smaller than the original beam size upon exiting the electron gun.
The beam optical system includes a magnetic focus coil, quadrupole coils, and deflection coils. The X-rays penetrate an object (e.g., a patient) and are detected by a detector array 22. The detector array 22 and targets 14 are coaxial with, and define planes orthogonal to, the system axis of symmetry 28. The output from the detector array is digitized, stored, and computer processed to produce a reconstructed X-ray video image of a portion of the object.
In the chamber region upstream of the beam optical system, a diverging beam is desired. In the upstream region, the electron beam can advantageously self-expand due to its own space-charge. The self-expansion depends on the force created by the electron space-charge. By contrast, downstream from the beam optical system, a converging, self-focusing, beam is desired.
The vacuum chamber contains residual or introduced gas that ionizes in the presence of the electron beam, producing positive ions. While these positive ions are useful in the downstream chamber region where a converging beam is desired, in the upstream region they can detrimentally counteract the desired beam expansion. Unless removed by an external electrostatic field upstream, the positive ions become trapped in the negative electron beam, neutralizing the space-charge needed for the desired beam self-expansion. In fact, neutralization can destabilize and even collapse the beam.
The usual arrangement in prior art scanning electron beam scanners is to remove such positive ions by passing the electron beam axially through at least one ion clearing electrode ("ICE") located in the upstream region. The ICE is coupled to an electrode potential of about 1 kV, and creates a transverse electric field. The transverse field sweeps away the slowly moving positive ions, without disturbing the considerably faster moving electrons, which have been accelerated by some 130 kV.
In this manner, ICE's remove positive ions only from the upstream region, permitting positive ions to accumulate downstream from the beam optics system. Downstream, positive ions beneficially neutralize beam space-charge, which permits the beam's attractive magnetic field to converge and self-focus the beam. Thus, downstream, convergence depends on the magnetic field created by the electrons in the electron beam.
The result is a self-repulsive, de-focusing beam in the upstream region, and a self-focusing beam in the downstream region. Elements of the beam optical system then focus and fine tune the converged beam as it scans along the X-ray target, to produce a sharp reconstructed X-ray image.
The upstream and downstream chamber regions are segregated by a washer-shaped positive ion electrode ("PIE"), coupled to a high positive potential, e.g., 2 kV. The PIE creates a large axial field that prevents positive ions (formed downstream) from migrating upstream, where their presence would be detrimental. Near the intersection of the upstream and downstream regions, there is usually placed a magnetic solenoid focus coil that provides and fine tunes the beam focus in response to a varying coil current.
While magnetic solenoid focus coils are used in the prior art, it is advantageous to reduce the number of components, including such coils, in a scanning electron beam CT system.
In a scanning electron beam CT system, there is a need for a mechanism that can focus the electron beam, and for a means for adjusting such mechanism, that obviates the need for a magnetic solenoid focus coil. Further, there is a need for an electron beam focus mechanism that permits dynamic focus fine tuning during a scan.
The present invention discloses such a mechanism.